The present invention is directed to obtaining highly precise position, velocity, time and attitude measurements and their time derivatives by the use and processing of multiple signals separated in frequency and by the use of these signals and their sum and difference components. One application of this multiple signal measurements technique is in the resolution of the integer cycle ambiguities associated with precise carrier phase measurements of the signals used in satellite navigation systems such s the U.S. Global Positioning System (GPS), or the Russian Global Orbiting Navigation Satellite System (GLONASS), or other systems. One implementation of this multiple signal technique is to use dual or xe2x80x9csplit spectrumxe2x80x9d signals that involves a moderate frequency separation of the signals (or signal energy), and employs an additional signal or signals separated by a greater amount(s) to provide for the progressive resolution of the integer cycle wavelength ambiguities associated with progressively more narrow lane widths (or difference frequency wavelengths). This process continues until the relative phase of the carrier itself is measured and the integer cycle wavelength ambiguities of the carrier signals are also resolved. The technique of the invention involves a signal structure with three or more signal components normally operating in one or more of the bands assigned to GPS, GLONASS or other systems. These signals are used in combination with one or more additional signal(s) at frequencies substantially separated from the dual, or split spectrum, signals. This approach provides significant performance improvements over conventional implementations and can be configured in various ways. The system performance improvements compared to currently available systems include improved accuracy, integrity, availability, continuity, and reductions in the time intervals required to obtain a navigation (or related) determination and in the capabilities of the user equipment to operate dynamically, and/or in a signal interference environment.
The United States, the Russian Federation and others (including the Europeans) have established, or plan to establish, orbiting satellite navigation systems. The GPS system, the GLONASS system and other systems, employ constellations of orbiting satellites which transmit signals to receivers on the earth (ground, airborne, marine) and in space which are used to determine precise three-dimensional position, velocity and time (e.g., latitude, longitude, altitude, 3D velocity and time) and in some cases angle (e.g., vehicle attitude) as well as differences and time derivatives of these parameters. Such signals can be used, for example, for navigation, surveying, timing, positioning and for measuring dimensional and other changes over time. Both the GPS and GLONASS systems use two separated bands of frequencies in the L-band portion (xcx9c1-2 GHz) portion of the electromagnetic spectrum. These bands have been allocated for radionavigation satellite services by the International Telecommunications Union (ITU).
In the case of both the GPS system and the GLONASS system, the frequency bands are designated L1 for the higher frequency band and L2 for the lower frequency band. A detailed description of the signal structure used for the GPS system is provided in Kayton, M. and W. R. Fried, Avionics Navigation Systems, 2d Ed., Chapter V, Satellite Radionavigation by A. J. Van Dierendonck, Section 5.5.5 GPS Signal Structure, pp. 213-282, John Wiley and Sons, Inc., New York, N.Y., 1997, which description is hereby incorporated by reference herein.
Referring to the drawings, FIG. 1 shows the existing GPS signal structure, generally designated by reference numeral 10. In FIG. 1, C/A designates the existing GPS coarse/acquisition code modulation on the L1 carrier, while P/Y indicates the GPS precise/encrypted code modulation of the L1 and L2 carriers, and L2xcfx86 indicates the xe2x80x9ccarrier phasexe2x80x9d part of the P/Y-code signal at L2 that is authorized for civil use (for ionospheric correction).
For the L1 band, the signal energy of the C/A-code is concentrated at the center of the bands 12, with very little C/A-code energy at or near the P/Y-code nulls 14,16. For the L2 band, there is no C/A-code signal centered in the band 18 and no C/A-coded signal at or near the P/Y-code nulls 20,22.
Throughout the drawings, the frequency occupancies of the bands (to their first spectral nulls) are shown, not the shape of the waveform, or signal power distribution, of each band. Those skilled in the art who have reviewed the present disclosure will readily appreciate the waveform shape in each situation.
Known systems have a number of drawbacks including the following: First, civil (Standard Positioning Service, or SPS) accuracy for differential systems using C/A-code corrections is normally to within several meters. To obtain accuracy within centimeters or decimeters adds considerable cost and complexity to the user equipment and is reliably achieved only by the use of techniques involving differential measurements of the carrier phases of the received signals. One problem in achieving high accuracy is the need to resolve the integer cycle wavelength ambiguity associated with the carrier phase measurements. To accomplish this with the current signal structure now requires the use of sophisticated and expensive software processing, moderate to long observation periods for high accuracy, statistical estimates of the probabilities associated with the observations and careful measurements of the effects of the troposphere, the ionosphere and other error contributors on the signals, especially at large differential system (reference to rover) separation distances. Second, the signal modulations currently provided for civil and military uses (e.g., C/A-codes and P/Y-codes) have maxima near one another or are collocated in frequency (e.g., , both the C/A-codes and the L1 P/Y-codes for GPS are centered at the GPS L1 center frequency). This arrangement is undesirable for some military purposes as well as for some civil applications.
It is therefore the object of the invention to improve accuracy and other performance characteristics at a moderate cost.
It is a further object of the invention to separate the signals available for civil use from the maxima of the signals for military use. This can be accomplished by (a) moving the civil signals away from the center of the band if the planned military signals (Lm) are to occupy the center of the band, or (b) moving the planned military signals away from the center of the band if the existing and planned civil signals are to be in the center of the band. While the first option (a) will be disclosed in detail, either can be used.
To achieve the above and other objectives, the present invention improves position, velocity, time and angle (attitude) determinations obtained by user equipment receiving radionavigation satellite (or other) signals by establishing and exploiting a new signal structure. This signal structure provides a number of features including means for rapidly and accurately resolving the carrier cycle integer ambiguities in the use of the signals for carrier phase measurement applications. This is accomplished by user systems using three (3) or more signals obtained from four (4) or more satellite (or other) signal transmitters. Specifically, addressing the use of the designated signal structure with GPS signals (and applicable to GLONASS and other signals), the existing signal structure for the GPS L1 band or the L2 band, or both, is modified to use dual (or split spectrum) signals. One representative implementation of the technique would be to use a pair of GPS coded signals (such as coarse acquisition, or C/A-code, signals, or other coded signals) at, or near (within several MHz, e.g., 2 to 6 MHz) of the P/Y-code nulls. The P/Y-code nulls refer to the GPS precision coded (PPS or P-coded) signals, with bi-phase modulation of the GPS carriers at bit rates of 10.23 Mbps for GPS and 5.11 Mbps for GLONASS. For GPS, the Y-code is the secure version of the GPS P-code, and is at the P-code bit rate. The first nulls of these codes occur at a frequency offset above and below their carrier center frequencies by the code bit (or xe2x80x9cchippingxe2x80x9d) rate and these frequencies thereby constitute a first lower null and a first upper null. In the case of the L2 band, a first coded signal could be located near (or at) the frequency corresponding to the lower P/Y-code null and a second coded signal could be located near (or at) the frequency corresponding to the upper P/Y-code null. In the case of the L1 band, a third coded signal could be located near (or at) the frequency corresponding to the lower P/Y-code null and a fourth coded signal could be located near (or at) the frequency corresponding to the upper P/Y-code null. This provides an arrangement in which the signals are separated in frequency such that the wavelength of the difference frequency between the signal frequencies at L1 and the signal frequencies at L2 (the wavelength of said difference frequency is the xe2x80x9cwide-lanexe2x80x9d) are of a dimension large enough to be resolvable by other available measurements. For example, if the sets of signal frequencies (the dual, or split spectrum signals) at L1 and at L2 are separated by 20 MHz, the difference wavelength, or wide-lane, is c/20 MHz (where c is the speed of light: xcx9c3xc3x97108 meters per second), or 15 meters. Similarly, for a third frequency separated from one of these signals by 50 MHz, then the wide-lane wavelength is about 6 meters in dimension. The cycle ambiguity of the 6 to 15 meter wide-lane is resolvable in user equipment by the use of conventional code (C/A-code or P/Y-code) processing capabilities of GPS, GLONASS and other systems, either on a stand-alone basis or by employing code differential techniques.
The initial wide-lane for each signal can be established through the use of the dual, or split spectrum, signal structure described. Resolution of the ambiguities of the wide-lanes are accomplished by the use of straightforward code, or differential code, ranging techniques similar to the current methods for obtaining one to six meter accuracy for GPS, GLONASS or other systems. Phase measurements at the wide-lane signal wavelength are then obtained for each of the satellite (or other transmitted) signals that correspond to the observer position. Then a more widely spaced set of two signal frequencies transmitted by the satellites (or other transmitters) is selected that provides a difference signal with a narrower lane width (or difference frequency wavelength).
For example, the separation between the GPS signal frequencies centered at L1 and L2 (xcx9c358.82 MHz) could be used to provide a narrow lane (wavelength) of about 86 cm. Phase measurements of the wide lane (15 m.) signals allows the resolution of the ambiguities of the narrow lane (86 cm.) signals. A set of phase measurements is then made of the difference frequencies forming the narrow lanes (the 86 cm lanes). At this point, direct measurements can be made of, for example, the GPS L1 carrier frequency signal phases that have a wavelength (at 1575.42 MHz) of about nineteen centimeters. The integer ambiguities of the L1 carrier signal wavelengths (19 cm.) are resolved by the use of the phase measurements of the narrow lane (86 cm.) signals. Relative phase measurements of the carrier signal wavelength provides a final high precision ranging measurement. For example, a three degree relative phase resolution of the carrier wavelength measurement provides a ranging precision of about two millimeters.
In this exemplary configuration, a coded signal could also be located (or retained) for backward compatibility purposes at the carrier frequency corresponding to the current C/A-code of the GPS signal in the L1 band.
The present invention offers the advantage of permitting GPS and other users to obtain accurate position, velocity, time, attitude and other information, from measurements obtained between a user, a differential reference receiver and a set of spacecraft emitters (such as GPS Satellites), or other emitters, ioncluding ground-based emitters. These measurements are of range, range difference, range rate (singly or in combination), differential carrier phase and phase differences, using three or more separate signals operating at differing frequencies such that the signals provide a means for establishing the range, range difference, carrier phase and phase difference as well as the integer cycle ambiguities associated with measurements of the relativecarrier phase of the signals. The multiple, step-wise resolution of the differential carrier phase integer cycle ambiguities is a significant aspect of the present invention.